This invention relates to implements with abrasive compacts and more particularly to techniques for fabricating such cutters from such compacts and for fabricating drill bits of such cutters.
A cluster compact is defined as a cluster of abrasive particles bonded together either (1) in a self-bonded relationship, (2) a means of a bonding medium disposed between the crystals, (3) by means of some combination of (1) and (2). Reference can be made to U.S. Pat. No. 3,136,615, U.S. Pat. No. 3,233,988 and U.S. Pat. No. 3,609,818 for a detailed disclosure of certain types of compacts and methods for making same. (The disclosures of these patents are hereby incorporated by reference herein.)
A composite compact is defined as a cluster compact bonded to a substrate material such as cemented tungsten carbide. A bond to the substrate can be formed either during or subsequent to the formation of the cluster compact. Reference can be made to U.S. Pat. No. 3,743,489, U.S. Pat. No. 3,745,623 and U.S. Pat. No. 3,767,371 for a detailed disclosure of certain types of composite compacts and methods for making same. (The disclosures of these patents are hereby incorporated by reference herein.)
Brazing is defined as a group of welding processes wherein coalescence is produced by heating to suitable temperatures above 800.degree. F. and by using a filler metal having a melting point below that of the base metals. The filler metal is distributed between the closely fitted surfaces of the joint by capillary action.
A brazing filler metal is defined as a metal or alloy to be added when making a braze and having melting temperatures above 800.degree. F. (but below those of the metals being joined).
Conventional rotary drill bits for oil and gas well drilling core drilling have heretofore used cutting elements such as (1) steel teeth, (2) steel teeth laminated with tungsten carbide, (3) a compact insert of cemented tungsten carbide and (4) natural diamonds all of which are set or molded in a tungsten carbide crown or cone. Due to the relatively short life and/or high operating cost of these conventional designs, it has recently been proposed to use synthetic diamond compacts as the cutting element in such drills.
In one prior art design, a drill bit cutting element is formed by attaching a diamond composite compact with a cemented carbide substrate by brazing the carbide substrate to a cemented carbide pin. The pin is mounted in a hole in the drill crown. The diamond layer is generally oriented in a radial sense to the center of rotation of the drill bit and penetrates the rock essentially as a cutting tool in a similar manner to a cutting tool which is used to cut metal on a lathe.
In a second prior art design, a cutting element is formed by furnace brazing a diamond composite compact in a recess of the crown of the drill bit. A portion of the compact is extended beyond the outer surface of the crown and forms a cutting edge for the drill.
One problem which has been encountered in field tests of the second design is that the stresses on each cutting element is severe and some disattachment of the cutters has been encountered. The stresses are caused because the structure of most rocks is heterogeneous and thus have layers of varying hardness. These layers cause a large variation in the impact loads to be applied to the cutting elements during drilling, and thus, the bond strength of such designs is not always strong enough to withstand such a widely varying impact loading.
Available attachment techniques and acceptable brazing filler metals for use with a diamond composite compact made in accordance with the teaching of U.S. Pat. No. 3,745,623 are limited because the diamond layer of such compacts is thermally degraded at temperatures above approximately 700.degree. C. Similarly, it has been found that a cubic boron nitride (CBN) composite compact made in accordance with the teaching of U.S. Pat. No. 3,767,371 and U.S. Pat. No. 3,743,489 is also thermally degraded at temperatures above approximately 700.degree. C. Thus, such compacts have been restricted from use in applications thought heretofore to require (1) the bonding of the compact to a support by a brazed material with a melting point close to or above the thermal degradation point of the compact or (2) the molding of the compact in an abrasion resistant matrix requiring high formation temperatures (e.g., cemented tungsten carbide) such as is commonly used in a surface-set rock drill crown.
Because of the thermal degradation problem, it has been necessary to use brazing filler metals with a liquidus below 700.degree. C. Such metals formed braze joints generally of lower strength than braze filler metals having a higher liquidus. Even when the lower temperature liquidus metals (such as BAg-1-ASW-ASTM classification) are used, temperatures approaching those at which the diamond layer is degraded are required; hence, great care is required to prevent degradation of the compact during brazing.
One brazing process which has been used with some success with the second prior art design is comprised of the following steps: (1) positioning a plurality of diamond composite compact cutters, respectively, in a plurality of recesses in a preformed, high temperature drill crown matrix (e.g., of tungsten carbide), weighing between about 90 to 136 kg., with a wire or powder of the braze filler metal in the bottom of the recesses between the surfaces to be joined, (2) placing the drill crown with the cutters clamped in position in a furnace, (3) heating the furnace and its content of "heat-up" cycle of approximately 8 hours to bring the furnace and its contents to a temperature between 690.degree. and 700.degree. C., (4) maintaining this temperature for a "soak" cycle of approximately 1 hour and (5) removing the bit from the furnace and allowing it to cool.
The long heat-up cycle is required to permit good heat distribution thereby promoting the formation of a strong bond and to allow the brazing temperature to be approached slowly so that the degradation temperature of the compact is not exceeded. However, heat-up cycles of such length with low temperature liquidus brazing filler metals are disadvantageous because they contain volatile components, such as cadmium and zinc, which give the metal its low liquidus temperature. Cadmium and zinc are volatilized from such metal as a function of time at temperatures over 350.degree. C. With the volatilization of these components the liquidus of the alloy is increased resulting in the poor wetting and weak bonding between the surfaces to be joined.
Accordingly, it is an object of this invention to provide a method for fabricating a drill bit providing improved bonding between the drill crown and a composite compact cutter.
Another object of this invention is to provide a fabrication technique permitting the use of a high formation temperature drill crown matrix.
Another object of this invention is to provide a drill bit fabrication method, which permits a shorter furnace brazing cycle to be used thereby reducing degradation of the filler metal and a risk of degradation to the composite compact.
Another object of the invention is to provide an improved composite compact which can be more easily brazed with increased bond strength to a tool or implement.